division of
the cytoplasm (although this doesn't necessarily occur in all species).

III. Prokaryotic (bacterial) cell division
Bacterial cells divide by a process called binary fission, which literally means
to split in half. The process is much simpler than in eukaryotic cells,
because prokaryotes (1) lack a nucleus and (2) have less genetic information
(DNA). In fact, bacteria have a single circular strand of DNA. This
"chromosome", which is about 500 times longer than an individual cell, must be
folded up to fit inside the cell. The chromosome is attached to the cell
membrane. After the DNA is
duplicated, the newly formed strand is also attached to the membrane. As the
cell enlarges it pulls the two chromosomes apart and then the cell pinches off.
See diagram in text.

IV. Eukaryotic cell division

A. The Cell Cycle
Cells progress through an orderly, predictable series
of events that include growth and division. These phases can be depicted as
follows:
interphase

Interphase.
The phase of the cycle during which the cells grows and carries out its
normal activities is termed interphase. Three major events occur during
interphase are: (1) G1, which stands for the first gap phase. During this
portion of the cycle, the cell grows in size, carries out its normal
activities and prepares to replicate (make a copy) the DNA; (2) S, which
stands for "synthesis", is the phase of the cycle during which DNA is copied
(replicated). Note that there is now twice as much DNA as there was at the
begining of interphase; and (3) G2, or the second gap phase. During G2 the
cell prepares for division. Thus, we can modify our cell cycle:
G1
→S
→G2
→nuclear division (mitosis/meiosis)
→
cytokinesis
→G1
→
and so on...

During interphase, the centrosome (which is also called
the microtubule organizing center - MTOC) divides as do the centrioles (if
present, as in animal cells). This region will be the site of new
microtubule formation and serves to anchor the microtubules.

Nuclear
division by mitosis or meiosis. This is the phase during which the nucleus
divides and the genetic information is parceled out into newly forming
daughter cells. Mitosis results in daughter cells with the same chromosomal
number as the original (parent) cell, whereas meiosis results in daughter
cells with half the original number of chromosomes. Meiosis is restricted
to sex cells for gamete production, whereas mitosis is responsible for
virtually all other cell divisions (i.e. for growth and repair). We'll
first focus on mitosis which occurs in several stages.

Prophase.
Chromosomes become visible, or in other words they condense. During
interphase the DNA strands are uncoiled (uncondensed) in the nucleus.
This uncondensed genetic material is called chromatin; thus an
uncondensed chromosome can be called chromatin. The process of
condensing is similar to how a rubber band on a balsa wood airplane gets
fatter as the propeller is twisted 'round and 'round. The condensed
DNA, and associated proteins, become the chromosomes.

Chromosome structure. The two halves of the chromosome
are termed chromatids. They are attached at a constricted region (centromere).
At the centromere, there are specialized sites, termed the kinetochore,
where microtubules will join with the chromosome.

In addition, during prophase, spindle microtubules
begin to form near the nucleus on opposite sides.

During late prophase, sometimes considered a separate
stage termed prometaphase, the chromosomes become fully condensed, the
nuclear envelope begins to disappear, and the spindle, which is a
football-shaped aggregation of microtubules, develops. The microtubules
protrude through the nucleus and begin to attach to the kinetochore
regions of the chromosomes. Also, the MTOC centers/centrosome
regions/centrioles their migration to the poles.

Metaphase. The nuclear envelope is gone and the chromosomes have
aligned along the equator, central axis, of the cell.

Anaphase. The chromosomes are moved to the poles at a rate of about 1 μm/min. Two things are responsible for this movement: (a) microtubules
attached to the kinetochores (kinetochore microtubules) become shorter.
Apparently they disassemble at the end near the kinetochore moving the
chromosomes closer to the pole; and (b) the cell expands because of the
force of spindle microtubules.

Cytokinesis.
This is the stage during which the cell physically separates into two. In
animals, the cell "pinches" in two. This occurs in a fashion similar to
closing the drawstring on a change purse. A series of actin microfilaments
just beneath the cell membrane serves to divide the cell in two along the
cleavage furrow. In plant cells, which have a rigid wall, they produce
another wall between the two new daughter cells. The newly formed wall is
called the cell plate. The position of the new wall is determined by a band
of microtubules that rings the cell prior to division, called the
preprophase band of microtubules.

V. Control of
Cell Division

A. Checkpoint Control
In order to divide, the cell must pass a certain "start" or checkpoint.
Once past, the cell is committed or obligated to complete the division process.
For most cells, this start point is the transition between G1 and S phases.
Thus, non‑dividing cells are typically arrested in G1 and can temporarily or
permanently enter a noncycling state called G0. There are other controls
in G2 (prevent cell from entering mitosis until DNA replicated) and metaphase
(prevents anaphase from beginning until chromosomes are aligned). Start point controls
include:

Cell Size, nutrients, growth factors, undamaged DNA.
The cell must reach a certain size or nutrient level, etc to pass the restriction. Thus cells
divide when they reach a certain size which is smart because it prevents
cells from getting too large or dividing if they are too small, or have too
few nutrients etc.;

Contact
Inhibition. For example, normal animal cells in culture will grow until
they form a single and contact other cells. Interestingly, cancer cells
have lost this ability.

Injury (or
other environmental factor). For example, liver cells don't normally
divide, but can be stimulated to do so by injury; and

Cells Can
Count. Normal mammalian cells in culture will only divide about 20‑50
times (sometimes called the Hayflick limit), then the population of cells
will die. Thus, there must be some type of "counting" mechanism. Many
types of cancer cells apparently can't count. HeLa cells are cancer cells
that have divided many times since they were first isolated from Henrietta
Lacks in 1951.

How do cells count? The answer seems to lie in
the telomere, which is a cap found on the end of each chromosome. Like
the plastic cap on your shoelace that prevents the shoelace from unraveling,
the telomere protects and stabilizes the chromosome. The telomere
region is made of from 1500 - 1600 nucleotides. The human telomere
region is characterized by the repeated sequence of nucleotides, TTAGGG.
It has been demonstrated that the telomere region looses 50 - 200
nucleotides during every division. Thus the shortening of the telomere
may be the actual abacus on which cells are counted; when the telomere gets
too short, the cell no longer divides. Interestingly, cancer cells
have shorter, but stable sized telomeres. But, these cells also
possess an enzyme (telomerase) that makes or lengths this region. In
malignant cells that were tested, 90 of 100 had the enzyme telomerase and in
immortal cells like HeLa cells, 98/100 have active telomerase. In
contrast, telomerase is only found in normal cells before birth.

To summarize, cancers
can be caused by, among other things, cells loosing contact inhibition,
failing to stop dividing when they should, and when the normal cycle occurs
faster than it should.

Chemical/Hormonal Regulation. There are apparently chemicals that
signal the cell to divide. For example, platelet-derived growth factor
(PDGF) is required for the growth of fibroblasts (collagen-producing cells)
in culture. PDGF is released upon platelet damage at a site of injury.
PDGF then stimulates fibroblasts to divide to repair the damage.

B. Cell cycle control
Once the cell begins to divide, the actions must be
carefully choreographed. At least two major type of proteins are important
in this process: cyclins and cyclin-dependent kinases (cdk's). Their role
is to phosphorylate proteins which, in turn, affects cell activity.

VI. Plants vs. Animals
The main difference in cell division between plants and
animals is cytokinesis (pinching vs. walling). In addition, animals have
centrioles but plants do not. These are apparently not an absolute necessity for
cell division.